3. Methods

Samples of the Champagne vent fluid were collected in special gas-tight, all-metal bottles constructed of titanium alloy. The bottles, which have an internal volume of ~150 ml, were initially evacuated. After the connecting lines were flushed, the bottle inlet was opened using a hydraulic actuator, and then hydrostatic pressure quickly forced the vent fluid sample into the bottle. At the end of the ROV dive, the samples were processed on board the ship using a high vacuum extraction line equipped with a low temperature (-60°C) trap and an all-metal bellows pump (Figure 5). The sample was first dropped from the gas-tight bottle into an evacuated glass flask containing ~1 g of sulfamic acid. The acid lowers the pH of the fluid, thereby aiding in the extraction of CO and other dissolved gases. The bellows pump was then used to pump the exsolved gases through the drying trap into a calibrated volume. After the pumping was completed, the total amount of gas was measured using a high precision capacitance manometer attached to the calibrated volume. Then splits of the dry gas were sealed into glass ampoules. For rare gas measurements, the ampoules were constructed of type 1720 or 1724 aluminosilicate glass with low helium permeability. During the 2004 R/V Thompson cruise, our extraction line had the capability of handling about 1.5 l of total gas, which proved to be inadequate for some of the very gassy vent fluid samples. Thus for most gas-rich samples it was necessary to carry out the extractions in multiple steps. While this provided an accurate assessment of total gas content, the multiple step extraction fractionated the samples, making them unsuitable for gas composition or isotopic measurements. For the 2005 R/V Natsushima cruise, the extraction line was fitted with an additional tank increasing the calibrated volume to 11 l, thereby allowing us to extract the samples in one step.

Figure 5. Schematic of the extraction line used for sample processing at sea.

For analysis of dissolved species, additional samples of vent fluid were collected in non-gastight PVC pistons with pressure relief valves at the top to capture the water component. Careful measurements using a temperature probe integral with the sampler inlet gave temperatures of 103°C for the most vigorous vents, although several other vents in the area were discharging fluids at temperatures between 11 and 68°C. Careful probing into the seafloor where the liquid droplets were forming found temperatures <4°C, consistent with the existence of CO in the liquid or hydrate state.

Sampling of the liquid droplets proved to be even more challenging. On one of the 2004 ROPOS ROV dives we collected about 0.5 l of the liquid CO droplets in an inverted plastic cylinder normally used for collection of sediment cores (Figure 6b), and observed the droplets as the submersible ascended to the surface at the end of the dive (Figure 6c). This was similar to an experiment conducted by Sakai et al. [1990a] in the Okinawa Trough. We were able to first observe the continuous conversion of liquid CO into white "sherbet-like" hydrate in the cylinder. Then as the submersible passed through ~400 m depth (at ~4°C), we observed rapid conversion of both liquid CO and hydrate into gaseous CO. This is precisely the pressure depth at which this phase transition was expected, thus confirming our hypothesis that the droplets were composed mainly of liquid CO (Figure 7). The plastic collection cylinder as well as some of the ROV camera face plates suffered permanent damage as a result of contact with the corrosive liquid droplets.

Figure 6. Photographs of sampling at the Champagne site in 2004 and 2005. (a) Fluid sampler being inserted into 103°C hydrothermal vent. (b) Droplets of liquid CO being collected in an inverted plastic cylinder held in the ROV arm. (c) Photograph of the plastic cylinder taken at about 400 m depth during the ROV's ascent to the surface. Most of the liquid droplets have converted to hydrate, and the hydrate is beginning to sublime into gaseous CO. (d) Close-up of the "droplet catcher" used during the 2005 expedition attached to the small volume gas-tight bottle. (e and f) The ROV Hyper-Dolphin sampling liquid CO with the droplet catcher and gas-tight bottle at the Champagne site.

Figure 7. Phase diagram for CO showing regions where solid, liquid, gas, and super-critical fluid (SCF) exist. P and T denote the critical pressure and temperature. The dashed line denotes the boundary of hydrate stability [Sloan, 1990]. The P, T conditions for the Champagne site liquid droplets and for the 103°C vent fluid are shown.

In 2004, liquid CO droplets were also collected by gluing a length of PEEK™ tubing into the plastic cylinder mentioned above and connecting the other end to the inlet of a titanium gas-tight bottle. The ROV again collected about 500 ml of the liquid droplets by holding the plastic cylinder inverted above the buoyant droplet stream. Then the gas-tight bottle was opened, drawing liquid CO into the bottle. As a safety precaution, we opened the bottle several times before the ROV surfaced to allow gas to escape and relieve the internal pressure. Because there was a mixture of liquid CO, hydrate, and water in the bottle, this led to fractionation of the sample gas composition. Furthermore, this liquid droplet sample had to be extracted in multiple steps, leading to further fractionation. However, we were still able to confirm that the droplets consisted of >90% CO by volume.

During the 2005 R/V Natsushima cruise, we employed a new method for the collection of the liquid droplets that was very successful. One of us (C. Young) designed a special "droplet catcher" consisting of a conical metal spring that was connected with PEEK™ tubing to a special titanium gas-tight bottle with low (~10 cc) internal volume (Figure 6d). The spring coil was first filled with liquid droplets by holding it over a stream of CO droplets exiting the seafloor (Figure 6e). We knew from previous experience that the droplets are sticky and do not tend to coalesce. The droplets were visible through the spring coils but stayed in place inside the coils. Then the spring coil was compressed against a flat surface on the ROV, thereby expelling most of the excess water between the droplets. Finally, the small volume gas-tight bottle was triggered, drawing the droplet sample into the bottle. During the Natsushima cruise, we were able to collect 4 good samples of the liquid droplets with this technique. Subsequent analysis showed that each sample contained about 5 cc of liquid CO. This converted to about 7 l of gas at STP in the extraction line, which we were able to handle quite easily with our enlarged calibrated volume.

In both 2004 and 2005, water column samples were collected using a CTD rosette package. Plume identification was accomplished using both a light scatter sensor and an Eh sensor. Samples were collected into Niskin type bottles and sub-sampled for helium isotopes, CO, and other plume components.

He and Ne concentrations, He/He ratios, and C/C ratios were determined by mass spectrometry, while CO, CH, H, and other gas concentrations were determined by gas chromatography. Total CO in the water column plume samples was analyzed by coulometry. Radiocarbon was measured on selected samples at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory. Hydrogen sulfide was analyzed on fluid samples collected with the nongas-tight PVC pistons using the conventional methylene blue method. However, due to possible gas loss from the PVC pistons, these HS values represent only a lower limit. As an alternative, splits of the gases from selected gas-tight bottle samples were sent to Atmospheric Analysis and Consulting (AAC), Inc., Ventura, CA, for analysis of reduced sulfur compounds by sulfur chemiluminescence (method ASTM D-5504). In addition, AAC measured CO abundances by conventional thermal conductivity gas chromatography on the same samples, thereby producing a measure of the HS/CO ratio.